The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of satellites and corresponding ground stations. Currently, approximately twenty-four satellites are used in the GPS. Each satellite continually broadcasts its location in space along with the current time from an internal clock. GPS receivers are able to determine their position by receiving and analyzing signals transmitted from the satellites. Two-dimensional locations are able to be determined by analyzing signals from three satellites, and three-dimensional locations are able to be determined by analyzing signals from four or more satellites. A GPS receiver determines its location by determining its distance from the GPS satellites based on the received signals and then performing a geometric triangulation on these distance measurements. GPS will be described in more detail below.
Although the current GPS has been successful, it has several shortcomings that affect the accuracy of positioning calculations. For example, GPS satellite signals are subject to errors caused by ionospheric disturbances and satellite orbit discrepancies. Ionospheric and tropospheric refraction can slow satellite signals and cause carrier signals and codes to diverge. Because ionospheric disturbances vary greatly from location to location, these errors are difficult to correct with civilian-type GPS receivers. These and other errors are described in more detail below.
Differential GPS (DGPS) can improve the accuracy of position measurements. DGPS uses an extra stationary receiver at a known location as a reference point. The stationary receiver measures GPS signal error by comparing its exact, known location with the location derived from the GPS signals. The reference receiver sends timing error measurements to mobile GPS receivers that allow these GPS receivers to correct for errors and get a more accurate position measurement. DGPS assumes that the reference point and other receivers will encounter similar errors. One example of DGPS is the Radio Technical Commission for Maritime (RTCM) Services, provided by the U.S. Coast Guard, which provide DGPS correction signals.
Space Based Augmentation Systems (SBAS) have been developed to further account for errors and better improve the accuracy, availability and integrity of the GPS. Wide Area Augmentation System (WAAS) is one type of Space Based Augmentation System (SBAS) used in North America. The Federal Aviation Administration (FAA) developed and uses WAAS to aid in landing aircraft. The FAA also has plans to develop a Local Area Augmentation System (LAAS) with reference receivers located near runways to further aid in landing aircraft, particularly in zero visibility conditions. One benefit of WAAS is that it provides extended coverage both inland and offshore compared to a land-based DGPS. Another benefit of WAAS is that it does not require additional DGPS receiving equipment.
Other governments are developing SBAS. In Asia, the SBAS is referred to as the Japanese Multi-Functional Satellite Augmentation System (MSAS). In Europe, the SBAS is referred to as the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world likely will have access to precise position data using these and other SBAS systems.
As will be described in more detail below, the WAAS is based on a network of wide area ground reference stations (WRSs) that are linked to cover a service area including the entire U.S. and some areas of Canada and Mexico. The number of WRSs is currently about twenty-five. The WRSs are precisely surveyed so that the exact location of each WRS is known. Signals from GPS satellites are received and analyzed by the WRSs to determine errors in the signals, including errors caused by the ionospheric disturbances described above. Each WRS in the network relays its data to a wide area master station (WAAS) where correction information is computed. The WAAS calculates correction messages for each GPS satellite based on correction algorithms and assesses the overall integrity of the system. The correction messages are then uplinked to Geostationary Communication Satellites (GEOs), also referred to herein as SBAS satellites or more particularly as WAAS satellites, via a ground uplink system (GUS). The SBAS satellites broadcast the messages to GPS receivers within the coverage area of the SBAS satellites on the same frequency as the GPS signals (e.g. L1, 1575.42 MHz). GPS receivers with SBAS capabilities are capable of using the SBAS correction message to correct for GPS satellite signal errors caused by ionospheric disturbances and other inaccuracies. The SBAS satellites also act as additional navigation satellites for the GPS receivers, thus, providing additional navigation signals for position determination.
With respect to WAAS, the correction messages currently are uplinked to two WAAS satellites. In the future, additional WAAS satellites are intended to be incorporated in the system. The GPS receiver is capable of being positioned within the coverage area of both of these WAAS satellites such that the receiver is capable of receiving WAAS correction messages from any one or both of these WAAS satellites. Additional GEOs are capable of being used for a more comprehensive SBAS that provides a larger coverage and more redundancy. As such, a GPS receiver is capable of being positioned within the coverage area of two or more of these SBAS satellites such that the receiver is capable of receiving SBAS correction messages from any one of these SBAS satellites or from two or more of these SBAS satellites. Multiple WAAS satellites may be available in the future as potential sources of correction information.
The WAAS satellites broadcast several types of correction messages, and the information contained therein requires a substantial amount of memory in a GPS/WAAS receiver. In order to minimize the amount of memory required to store WAAS correction information, when multiple (i.e. two or more) WAAS satellites are available to be used by a GPS/WAAS receiver, it is desirable to obtain information only for the satellite that is the most reliable source of this information. The accuracy, desirability and/or equivalency of SBAS correction messages are not necessarily the same for the various SBAS correction sources. Accordingly, there exists a need for an improved method and system for determining the appropriate or desired geographical correction source for SBAS corrections and which benefits from the SBAS data while using a minimal amount of memory and system resources.